The disclosure relates to aircraft flight control systems and specifically to the automatic control of an aircraft's flight path. Automatic pilot systems are widely used in the aviation industry to control the flight path of an aircraft. Conventional control systems typically utilize the elevator as the control surface for effecting changes in the aircraft's vertical path. One objective of the disclosure is to enhance the precision of vertical path control afforded by one or more conventional pitch control systems coupled with an automatic pilot system, both during landing and non-landing flight situations.
As a representative example, an automatic landing is a vertical path tracking task that requires precise vertical path control in order to achieve acceptable performance. Automatic landing capability is required for operations in the most severe low visibility weather, referred to as Category IIIB low weather minima, and is used in less restrictive weather minimums to enhance safety and reduce flight crew work load. An Automatic Landing System (ALS) provides the precise vertical and lateral path guidance necessary to meet the stringent performance requirements specified for low weather minimum operations.
The vertical path guidance provided by an ALS includes both glide path control and the flare maneuver. Precise control of vertical position relative to the desired vertical path is essential in order to achieve the performance required for Category III operations. The glide path provides the established descent gradient and longitudinal position reference for final approach flight path guidance. The flare maneuver provides the transition from the glide path to touchdown at the desired location on the runway. Ideally, the ALS will land the aircraft at the same point on the runway regardless of environment or facility. In other words, the design must be very robust given the wide range of environmental conditions, terrain, and runway characteristics that the aircraft will be subjected to during automatic landings. However, in practice the vertical path tracking provided by the ALS is significantly affected by shearing winds, terrain, and runway characteristics. Any enhancement of an existing autopilot design that improves vertical path tracking will reduce the impact of the aforementioned disturbances during automatic landing operations.
For automatic landings, the autopilot used in airplanes such as the 777, 757, 767, and 747-400 utilizes a vertical position control law design to generate an elevator command that provides glide path control and the flare maneuver. The elevator command is generated with an elevator vertical position feedback control system. The vertical position control law design produces a pitch attitude command that is proportional to the altitude error and altitude rate error and the integral of the altitude error. The error is defined as the difference between the command and the actual (as measured by a sensor). The pitch attitude command is converted into an elevator command that is used to move the elevator control surface. The design is tuned to provide accurate vertical path tracking with acceptable stability characteristics. One problem with relying solely on an elevator feedback control system for vertical position control is that high gains are usually required to achieve the desired vertical path tracking accuracy. However, excessively high gains in the elevator feedback control system can compromise the overall system stability, potentially resulting in interaction with aircraft structural modes. High gains in the elevator vertical position feedback control system can also result in excessive pitch activity that may be objectionable to the flight crew.
Autopilots typically utilize a predictive or feed forward elevator compensation of some sort in combination with elevator feedback control to achieve disturbance rejection. This combination of feed forward elevator compensation and elevator feedback control allow performance objectives to be met without resorting to excessively high and potentially destabilizing feedback gains. The types of elevator feed forward compensation utilized are typically either short term moment compensation or long term force compensation.
For short term moment compensation, a control surface command (elevator) is generated such that a moment is created that cancels the moment predicted to be generated by the disturbance. For long term force compensation, a pitch attitude command is generated to counteract the steady state trim changes due to a disturbance. Short term moment compensation tends to limit pitch attitude change in response to a disturbance, whereas long term force compensation tends to generate pitch attitude change in response to a disturbance. Short term moment compensation is used for balancing pitching moments due to changes in stabilizer, and changes in thrust and ground effects, but is not very effective for dealing with vertical path disturbance due to changing winds. Long term force compensation, on the other hand, is quite effective in countering the disturbances due to changing winds. However, during the flare maneuver, the pitch attitude changes resulting from long term force compensation tend to result in undesirable pitch activity from a fight crew acceptability standpoint.
During a landing there are also geometrical constraints that need to be considered. The pitch attitude of the airplane must be limited to prevent ground contact of the nose landing gear prior to the main landing gear and ground contact of the aft body (tail strike). While limiting the pitch attitude within the geometrical constraints reduces the probability of a nose gear first contact and tail strike during an automatic landing, the ability of the autopilot to maintain the commanded vertical path using a vertical position elevator feedback control system and feed forward elevator compensation can be diminished by these geometrical constraints. For example, during a flare maneuver, the geometrical constraints may prevent the autopilot from maneuvering as aggressively in response to the vertical path upset caused by shearing winds.
A method for reducing vertical position control errors of an aircraft is needed to decrease one or more problems associated with one or more of the existing methods.